page_title: São Miguel Island
section_title: Geology
hierarchical_section_title: São Miguel Island / Geography / Geology
caption_attribution_description: Português: Ginetes visto do alto da Caldeira das Sete Cidades (Portugal), Ilha de S. Miguel
context_page_description: São Miguel Island, nicknamed "The Green Island", is the largest and most populous island in the Portuguese archipelago of the Azores. The island covers 760 km² and has around 140,000 inhabitants, with 45,000 people residing in Ponta Delgada, the archipelago's largest city.
context_section_description: São Miguel is bisected by many faults from the northwest to southeast in the direction of the Terceira Rift, a triple junction of the Eurasian, African and North American tectonic plates.  This system is best expressed in the western part of the island with extensive geological formations, such as the Mosteiros Graben (along the western flank of the Sete Cidades Massif), the Ribeira Grande Graben (along the northern flank of the Água de Pau Massif), and the many cones and fissural structures along the interior of the island.  In the ancient crater of Furnas the faults are aligned west-northwest to east-southeast.  Zbysewsky (1959), among others (note references) identifies eight geomorphological structures on São Miguel that correspond to the formative features that built the island, including:
  The Sete Cidades Massif – an area that occupies the extreme western part of the island, and corresponds to a central volcanic crater and lake-filled caldera, with various cones, deposits of pumice, lava domes and maars.  In the northeastern flank of this volcano the Mosteiros Graben, a tectonic structure created from the collapse of lands and located along a northwest to southeast orientation.  Along other regional fractures and radial faults there are ancient spatter cones and lava domes;
  The Picos Volcanic System or Picos Region – is situated along a northwest–southeast alignment, and defines a range of spatter cones and relatively level ground between the Sete Cidades and the Água de Pau Massifs;
  The Água de Pau Massif –  this central feature corresponds to the central volcano on the island, and includes the Lagoa do Fogo (Lake of Fire), many lava domes and pumice cones.  On the northeastern flank of the Massif the Ribeira Grande Graben is visible, representing a tectonic depression oriented northwest to southeast;
  The Achada das Furnas Plateau – a region with a central plain marked by cones and maars, with deposits along a west-northwest to east-southeast and northwest to southeast;
  Furnas Volcano – located in the eastern part of the island, along the southern coast, and comprising two ancient calderas, occupied by a lake (Lagoa das Furnas).  Within the system one can find many pumice cones, maars and lava domes;
  Povoação Volcano – comprising a central caldera, generally well-eroded and whose southern rim has disappeared to the southern coast.  Within its interior, marked by several river-valleys and cliffs, are several spatter cones;
  The Tronqueira Region – it occupies the extreme easterly portion of the island and corresponds to a mountainous region, divided by many river-valleys that are usually delineated by tectonic fractures;
  The Northern Coastal Platform – located along the northeastern portion of the island, and marks a zone of relatively moderate topography, limited by the coast to the north and the northern crater rims of Furnas and Povoação volcanoes to the south.
  São Miguel comprises six volcanic zones, all are Quaternary in age except the last, which is partly Pliocene. From west to east these zones are: the trachyte stratovolcano of the Sete Cidades Massif; a field of alkali-basalt cinder cones and lava flows with minor trachyte; the trachyte stratovolcano of the Água de Pau Massif; a field of alkali-basalt cinder cones and lava flows with minor trachyte and tristanite; the trachyte stratovolcano of Furnas; and the Nordeste shield, which includes the Povoação caldera and consists of alkali basalt, tristanite and trachyte. Dormancy ages for these regions include: 400 year for Sete Cidades, 145 for zone 2, 1150 for Água de Pau, and 370 for Furnas, while eruptions in the Nordeste have not occurred in the past 3000 years.
  These geomorphological structures have resulted from millions of years of compound growth that began in the eastern portion of the island; around 4 million years ago the Nordeste Volcano burst from the ocean floor in effusive and fissural eruptions. These eruptions were composed of basaltic lava flows and spatter cones whose products reached a height of 1,100 metres (3,600 ft) forming the mountainou
caption: Ginetes, situated in the westernmost part of the island, near the coast.
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